Field of the Invention
This invention relates to an optical communications module having two sets of M (M;channel number) photodiodes and one set of M laser diodes, suitable for multichannel bidirectional optical communications systems which transmit a variety of optical signals by making use of two kinds of receiving signals λ2 and λ3 and one kind of a transmitting signal λ1 via M channel optical fibers. The optical communications module includes M laser diodes (LDs) for making and transmitting M channel signals of a λ1 band, M photodiodes (PD1s) for receiving M channel signals of a λ2 band and M photodiodes (PD2S) for receiving M channel signals of a λ3 band sealed in a package. The module has 3M optoelectronic device chips (M LDs+2M PDs). There is no device containing so many optoelectronic chips in a package yet.
This application claims the priority of Japanese Patent Application No.2002-176524 filed on Jun. 18, 2002, which is incorporated herein by reference.
A single channel bi-directional optical communications system makes use of a single optical fiber for transmitting downward and upward optical signals in two directions. An LD/PD module for the single channel bi-directional communications should have a laser diode of making transmitting signals, a photodiode of receiving signals and a signal dividing part (e.g., y-branch) for dividing propagating signals into transmitting ones and receiving ones. Requirements imposed upon the signal dividing part are small division loss, small optical crosstalk, weak electromagnetic and electric crosstalk.
Optical crosstalk means noise generation in a photodiode (PD) caused by invasion of strong light beams emitted from a laser diode (LD). The optical crosstalk is a serious problem for simultaneous bi-directional communications. Transmitting signal wavelength λ1 emitted from the laser diodes (LD) is different from receiving signal wavelength λ2 which has been sent from another port (a subscriber or a station). Conventional photodiodes employed for communications modules have an InGaAs light receiving layer (active layer) which has sensitivity within a wavelength range between 1.0 μm and 1.6 μm.
The InGaAs photodiodes sense both λ1 and λ2. Sensitivity of λ2 is a reason for causing optical crosstalk in a simultaneous bi-directional LD/PD module.
Another problem is electrical crosstalk among laser diodes. Strong current for driving lasers leaks in a package and a bench. The strong noise current has an influence upon the driving state of neighboring laser diodes.
Besides, there is also electromagnetic crosstalk among laser diodes and photodiodes. Noise current generates electromagnetic waves which fly in space to the photodiodes of high impedance in the same package and perturb the actions of the photodiodes.